I many machines, it is important, form the safety point of view, that in a reliable way to know where a moving machine part is and also how fast it moves. For an industrial robot, for example, it is used to divide operation area into safe sectors. This means that if a person walks into the protected sector, then the robot is stopped only if it is in the same sector by using a safety device which detects if the person enters the sector. The sector in which the robot operates is usually detected by double switches which are affected by ridges whose length correspond to a protected sector. If a robot, for example, turns so that it is heading into the sector in which the person is present, which is indicated by means of the safety device in the sector in question is affected. To set the size of the safety sector mechanical ridges are used. These ridges must be designed so that the robot can stop before coming into the safety sector. How long it takes for the robot to stop depends on the stopping distance at the worst case, i.e. the fastest speed of the robot. The longer the stopping distance a robot has the longer from the safety sector the robot must receive the stop signal.
All machine parts having turning or straightforward movements may need sector supervision from the safety point of view. Sector supervision may for example be used for sector safety.
The speed a machine part moves is also an important issue for the safety design.
If a machine moves with low speed there is a batter chance for a person to stand clear. Some times low speed in combination with a stop possibility using a switch bottom is a method that is used as safety measure, which is used for example at maintenance or fault-localization. An automatic speed monitoring may control that the moving machine part is stopped directly if the speed is exceeded. When designing the safety sectors one may monitor that the speed with which the machine goes from a safety sector to another must be low. This means that the addition of supplementary long stop distance at high speed can be eliminated, i.e., the machine is stopped in that time if the speed at such a position is too high. A condition to surely know where the machine is, demands that the area supervision fulfils the requirements which are expected, to be able to rely on the information.
EP 072866 relates to a robot control system for positioning the movable unit of a robot. The system comprises a hydraulic unit using direction solenoid valves. The system comprises a detecting means and a control unit for calculating the difference between the instructed position and the present position. The control unit controls the acceleration of the hydraulic fluid and measures it. When the difference between a second preset value becomes equal to a first preset value, a brake is operated to stop the unit. The system does not use two position detectors which provide the position of the moving part.
EP 0927612 relates to a robot controller, in which when a point representative of position of a robot makes an abnormal motion in space, such abnormality is sensed and the robot brought to a stop. A detector continuously gives the position of the robot which is compared to an abnormities position index. When an abnormal motion is detected, the robot is immediately stopped.